REMOTE FIELD EDDY CURRENT MILITARY AND COMMERCIAL PLATFORM

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REMOTE FIELD EDDY CURRENT MILITARY AND COMMERCIAL PLATFORM Powered By Docstoc
					   REMOTE FIELD EDDY CURRENT MILITARY AND COMMERCIAL
                 PLATFORM APPLICATIONS

                     Yushi Sun, Innovative Materials Testing Technologies, Inc.
       John C. Brausch, Kenneth J. LaCivita, Lt William Sanders, Air Force Research Laboratory


Introduction
A new development in NDI (nondestructive inspection) electromagnetic, eddy current technology enables
users to inspect conductive materials under thick layers of composite and find subsurface flaws in thick
aluminum, titanium and steel structure. The method is called Remote Field Eddy Current (RFEC). A
group of Scientists refined this technology after years of working with Flat Geometry Remote Field Eddy
Current (FG_RFEC) technique, as well as in finite element modeling of electromagnetic NDI phenomena
[1- 4]. A new eddy current instrument, Super-Sensitive-Eddy-Current (SSEC) system with extremely high
gain, 100 dB – 140 dB, has also been developed to deal with the low level signals obtained from the
FG_RFEC technique after deep penetration.

Conventional eddy current techniques, ECT, are capable of detecting only surface and subsurface flaws
due to restriction of the skin-depth equation. FG_RFEC technique with the SSEC system allows
measurement of signals that have penetrated through the whole wall thickness. Skin-depth is no longer the
limit in flaw detection. Meantime, the technique also has high sensitivity to surface and subsurface flaws
due to the high gain of the SSEC system.

In recent years, a number of possible applications of the FG_RFEC technique have been found in aircraft
and aerospace industries [5-13].

As the aircraft industry is driven to reduce structural weight, increase corrosion resistance, and cost
savings, more aircraft designs are utilizing composite materials attached to a metallic substrate of
aluminum and titanium. This design retains the strength and durability characteristics of traditional
metals, while reducing weight and increasing corrosion resistance. As these new materials are developed
for use on new aircraft platforms, so must the NDI technology be developed to inspect them. NDI is
necessary in the development, article testing and the operational phases of the aircraft structure. New
commercial aircraft platforms such as the Airbus A380, Boeing 787, are incorporating similar structure in
their designs. Existing military platforms also incorporate the composite/metallic structure and future
platforms will most likely use this type of structure. In some cases older commercial designs
incorporated nonmetallic barriers in fuel cells and other wet areas of the aircraft, to reduce corrosion and
seal fuel cells. Some of these areas may now, for the first time, be assessed during operational phases
utilizing the RFEC technology.

The paper presents some examples of remote field eddy current capabilities covering six new aircraft
applications: 1) Detection of aluminum layer cracks through one and half inch of polycarbonate; 2)
Detection of aluminum layer cracks through thick graphite epoxy composite; 3) Detection of titanium
layer cracks through thick graphite epoxy composite; 4) Detection of cracks in a titanium layer through a
thick graphite epoxy composite and suppression of sealant groove signals; 5) Detection of cracks 0.50”
below a aluminum top layer structure surface; 6) Detection of fine surface and subsurface cracking on
curved steel surface in a Airbus A-320 landing gear structure.
The next step is to assess existing or new inspections to find the right fit and application of this advanced
technology that is now available to our industry.

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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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FG_RFEC probes & SSEC system

There are two kinds of FG_RFEC probes often used for crack and corrosion detection: sliding probes and
rotational probes. Sliding probes are simple and easy in use, while a rotational probe, which is rotating
around a fastener, provides higher sensitivity to cracks under fasteners, because minimum signal from the
fastener appears in detected crack signals. Figure 1 shows typical FG_RFEC probes to be used in this
paper.




     Sliding Probe RF4 V.3A                      Sliding Probe RF2 V.3A


                              Slip ring & connector
                                                               Ball-bearing rotation
                                                               guide




                                        Probe Carriage
         Probe head
                                                                     Optical lens for
         Rotational Probe RF4 ROT with accessories
                                                                     centering

                     Figure 1: Typical FG_RFEC probes to be used in this paper
SSEC system is a modified version of a conventional eddy current (EC) instrument. It characterized by
the features below:

    1. High gain: up to 100 – 140 dB, and low S/N (signal to noise) ratio;
    2. Computerization: it utilizes a PC as its base;
    3. Compatibility with conventional EC probes. In other word, it works with any conventional EC
         probe. Conversion to conventional probe operation is achieved by changing the probe connector.
A photo of such a system working with a laptop PC is shown in Figure 2. The system consists of a
12”×8.75”×1.75” hardware box and installed software. The PC can be of customer selection: a desk-top
or a lap-top.



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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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    Laptop PC




                 SSEC RF02 +
                   Installed
           Figure 2: SSEC RF02 system with a laptop PC (left) and the System Screen (right).
Application No. 1: Detection of aluminum layer cracks through one and half inch of polycarbonate.

Specimens:
Three 7”×13” polycarbonate pieces of different thicknesses are used in this test. A 9.0”×1.25”×0.20”
aluminum strip is attached to the bottom center area of each of the polycarbonate pieces using six
titanium fasteners, 1.0” apart, as shown in Figure 3 A. Two though thickness EDM notches are made
between fasteners No.2 & No. 3, No. 4 & No. 5 on each of the aluminum strips.

The numbering and the thicknesses of the three polycarbonate specimens are listed below:
567-007: 0.567”; 0.484-007: 0.483” and 0.442-007: 0.442”.

Sliding probe RF4 V.3A scanning over different thicknesses of polycarbonate layers
At first an RF4 V.3A probe is placed on top of each specimen and scanning above the aluminum strip as
shown in Figure 4. The thicknesses of the polycarbonate layers are 0.442”, 0.483” and 0.567”,
respectively. To experimentally test the limits of the technology, we tried the inspection through two
layers, Figure 3 B. This results in three more data for thicknesses of 0.925” (0.442” + 0.483”), 1.009”
(0.442” + 0.567”) and 1.050” (0.483” + 0.567”). A signal magnitude – polycarbonate thickness related is
shown in Figure 5.

We also tried scanning the probe over all the three layers, Figure 3 C. The total thickness is 1.492”
(0.442” + 0.483” + 0.567”). The EDM signal is still clear as shown in Figure 6.




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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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             A




            B




            C




                 Figure 3: Specimens: A – single layer; B – Double layers; C – three layers.

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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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 Figure 4: A RF4 V.3A is scanning along the center-line of a polycarbonate layer(s) directly above the
                                          aluminum strip




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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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              Figure 5: Signal magnitude – Polycarbonate thickness relation at f = 2.0 kHz




Figure 6: EDM signals obtained when scanning over all three layers with a total thickness of 1.492” at f =
                                              200 Hz.

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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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Application No. 2: Detection of aluminum layer cracks through thick graphite epoxy composite

Specimens:
Four 7”×13” graphite epoxy composite pieces of two thicknesses, 0.520” and 0.896”, are used in this test.
A 9.0”×1.25”×0.20” aluminum strip is attached to the bottom center area of each of the composite pieces
using eight titanium fasteners, one inch apart, as shown in Figure 7 A. A number of though thickness
EDM (electro-discharge machined) notches of different lengths and with horizontal and vertical
orientations are made on each of the aluminum strips as shown on Figure 7.

Probe RF4 V.3A scanning over the four specimens
The four specimens were scanned using Probe RF4 V.3A. The test results are shown in Figure 8. Typical
data compared with noise level are listed in Table 1.

Note: In the detection of deep flaws, signal magnitude and S/N ratio, may not make sense in identification
of a crack. Phase angle, or signal orientation in impedance plane, some times plays a major role. We have
seen in Figure 8 that signals from horizontal notches tend to be oriented in the first quadrant, while signal
from vertical notches take directions around 180°.




       Figure 7: Two drawings showing four specimens with thick graphite epoxy composite layer

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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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           Figure 8: Impedance Planes obtained from scanning the four composite specimens
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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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                         Table 1: Typical Data Taken from the Curves in Figure 8




Application No. 3: Detection of titanium layer cracks through thick graphite epoxy composite

Specimens1:
Three, 0.25” thick titanium plates (see Figure 9) with fatigue cracks of different lengths, 0.250”, 0.500”
and 0.750”, respectively, combined with three graphite epoxy composite plate of different thickness,
0.250”, 0.333” and 0.500”, result in a matrix of nine specimens as shown in Table 2. A 5/16” titanium
fastener is used to tie a composite layer and a titanium plate together.

Rotational probe RF4 ROT scanning around the fastener of each specimen
All cracks in the nine specimens are detected with significantly high S/N ratio. A typical crack signal in
the impedance plane, provided detection at the smallest crack length, 0.25”, and the largest composite
thickness, 0.500”, is shown in Figure 10 A.

Summaries of the test results are in Table 1, in a text format, and in Figure 10 B, using curves.




1
Specimens are provided by US Air Force Research Laboratory.
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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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                            Figure 9: Typical configuration of a test specimen


                                                 Table 2




          A                                                B




 Figure 10: Impedance plane at 0.25” crack & 0.50” thick composite (A) & Summary of test results on
                detecting titanium layer cracks through graphic epoxy components (B)

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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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Application No. 4: Detection of cracks on titanium layer through thick graphite epoxy composite
and suppression of sealant groove signals

Specimens2:
A 0.500” long fatigue crack is generated from the center hole of a piece of 0.25” thick titanium plate. Two
sealant grooves of different orientations are made on the two sides of the plate, one side each, Figure 11.

Rotational probe RF4 ROT scanning around the fastener of each side of the specimen
Figure 12 shows the configuration of the scan. A special band-pass filter is used to minimize the signals
generated by the sealant groove. Typical signals detected from the two scans are shown in Figure 13. It is
clearly seen that the groove signals have been effectively suppressed.




            Figure 11: Titanium plate with a fatigue crack and two sealant grooves, one side each




    Figure 12: A Rotational probe RF4 Rot is used to detect the 0.500” crack in the titanium plate. A band-
                      pass filter is used to suppress the signals from the sealant grooves

2
Specimens are provided by US Air Force Research Laboratory.
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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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                                               Case 1: Horizontal Groove is on top




                                                                           Pickup


                                                Case 2: Vertical Groove is on top




                                                                              Pickup


     Figure 13: Groove signals have been effectively suppressed by a band-pass filter. The two left plots are
    impedance planes. The upper curve in each of the two right plots is the real component of the signal, the
                                  lower curve is the imaginary of the signal.

Application No. 5: Detection of crack 0.50” below aluminum structure surface

Specimen3:
A drawing of the specimen is shown in Figure 14. It is a two layer aluminum structure with a 0.200” long
EDM notch generated from a fastener hole of the second layer. The thickness of the first aluminum layer
is 0.500”.

Rotational probe RF4 ROT scanning around a fastener without an EDM notch and another fastener
with a 0.200”long EDM notch
The impedance planes obtained from the two scans are shown in Figure 15. The crack signal is
distinguished from the edge effect signal primarily by it shape instead of signal magnitude.

Note: In the detection of deep flaws, signal magnitude and S/N ratio, may not make sense in identification
of a crack. Phase angle, or signal orientation/shape in impedance plane, some times plays a major role.
Comparison of an unknown signal with a known cracked or no-crack signal may be a good way to
identify a crack. We have seen in Figure 15 the lower portion of the cracked signal, in green, has a similar
shape as the no-crack signal. The upper portion of the cracked signal, in white, is considerably different
from the no-crack signal. In other words, in the crack signal there is a significant part of the signal
appearing in the first quadrant of the impedance plane and significant positive real and positive imaginary
appearing in the X & Y strip charts.




3
Specimen is simulating Canadian aircraft CC130. It was provided by Captain D.J. Butcher, Canadian Air Force.
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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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                      Figure 14: A specimen simulating 2nd layer crack in CC130 structure




                  Figure 15: Detected crack signal, right, compared with no-crack signal, left


Application No. 6: Detection of fine surface and subsurface cracking on curved steel surface of
Airbus A-320 landing gear structure

Specimen4:
The specimen was a part removed from a retired Airbus A-320 landing gear, Figure 16. The surface is of
complex shape where it is difficult to apply a sliding probe for crack detection. A pencil probe is the one
applicable here, but the detection suffers from tremendous noise cause by hand shaking of an operator.




4
    Specimen is provided by Northwest Airlines.
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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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                                                          φ0.080” hidden hole
                                                          drilled from top cut
                                                          section


                                                        0.030”×0.030” upper EDM
                      Possible
                      crack area

                                                       0.030”×0.030” lower EDM




Figure 16: Specimen cut from Airbus A-320 landing gear with two 0.030” (L) × 0.030” (D) EDM notches
                            and a φ0.080” hidden hole on its curved surface

Sliding probe RF2 V.3 with a specially designed probe holder, Figure 17 A & B, used in scanning the
curved surface
The relative probe-holder position is adjustable. During a scan first to keep the sensitivity surface of the
probe having a good a contact with the surface under inspection. Then, adjust the probe holder position to
have at least one or two tips of the holder contacting the surface under inspection as well. After that the
probe can be moved smoothly along the surface within a certain distance, see Figure 17 C.

The detected crack signals and the signal from the hidden hole are shown in Figure 18-20.
   A                              C




       B




                                        A




Figure 17: Probe RF2 V.3 working with a specially designed probe holder scanning the curved surface of
                                 the A-320 landing gear specimen
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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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    Lower 0.030”×0.030” EDM
    F = 40.0 kHz




    Upper 0.030”×0.030” EDM
    F = 40.0 kHz




    Hidden hole
    F = 1.0 kHz
                  Figure 18: Signals obtained from the surface cracks and hidden holes

Summary
   1. Metallic layer cracks underneath thick composite layer are detectable using FG_RFEC technique
      – sliding probes or rotational probes;
   2. A groove signal can be effectively suppressed using a band-pass filter;
   3. FG_RFEC technique working with SSEC system is capable of detecting deeply hidden cracks
      and fine surface and subsurface cracking with high sensitivity.
   4. The next step is to assess existing or new inspections, now available to our industry, to find the
      right fit and application of this advanced technology.

Acknowledgements
The authors would like to express their appreciation and great thanks to Jeff A. Register and Michael J,
Fortman, Northwest Airlines5, and Captain D. J. Butcher, Canadian Air Force, for their providing test
samples, test requirements and comments to IMTT.

The authors would like also to express their appreciation and great thanks to Dr. Ward D. Rummel, D&W
Enterprises, LTD, for his careful view and valuable comments to this paper.


5
 Jeff A. Register and Michael J, Fortman were with Northwest Airlines when they were involved in the
work, Application No. 6, stated in this paper. Now they are working with Aerotechinics NDT, Inc.

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References:
[1]. Y. Sun, S. Udpa, W. Lord and D. Cooley, “A Remote Field Eddy Current NDT Probe for the Inspection of
Metallic Plates”, published on Topical Conference Paper Summaries Book - ASNT’s International Chemical and
Petroleum Industry Inspection Technology (ICPIIT) IV Topical Conference, 1995.

[2]. Y.S. Sun, S. Udpa, W. Lord, and D. Cooley, “A Remote Field Eddy Current Probe for the Inspection of Metallic
Plates”, Materials Evaluation, Vol. 54, No. 4, PP. 510-512, April 1996.

[3]. Y.S. Sun, W. Lord, L. Udpa, S. Udpa, S.K. Lua, K.H. Hg, and N. Nath, “Expanding The Remote Field Eddy
Current Techniques To Thick - Walled Aluminum Plate Inspection”, Electromagnetic Nondestructive Evaluation -
Studies in Applied Electromagnetics and mechanics 12, pp.145-152, Edited by T. Takagi, et al., IOS Press, 1997.
[4]. Yu-shi Sun, US Patent 6,002,251, “Electromagnetic-Field-Focusing Remote-Field Eddy-Current Probe System
and Method for Inspecting Anomalies in Conducting Plates”, Issue date: 12/14/1999.

[5]. Yushi Sun and Tianhe Ouyang, “Detection Of Cracks In Multi-Layer Aircraft Structures With Fasteners Using
Remote Field Eddy Current Method”, Proceeding of SPIE Volume 3994, Nondestructive Evaluation of Aging
Aircraft, Airports, and Aerospace Hardware IV, Chair/Editor Ajit K. Mal, 7-8 March 2000, Newport Beach, CA.

[6]. Yushi Sun, Tianhe Ouyang, and Satish Udpa, “Multi-Layer Aircraft Structure Inspection Using A Ultra-
Sensitive Remote-Field Eddy-Current System” , Review of Progress in Quantitative Nondestructive Evaluation, Vol.
20, Plenum, 2001.

[7]. Yushi Sun, Tianhe Ouyang, and Robert J. Lord, “Detecting 1st and 2nd Layer Simulated Cracks in Aircraft Wing
Spanwise Splice Standards Using Remote-Field Eddy Current Technique”, Proceedings of Aging Aircraft
Conference 2001, Kissimmee, Florida, September 10~13, 2001.

[8]. Yushi Sun, et al., “Building Calibration Standard Remote Filed Eddy Current Technique Detecting Deeply
Hidden Corrosion in Aircraft Structures”, 5th International Aircraft Corrosion Workshop Proceedings, Solomons,
MA, August 20-23, 2002.
[9]. Yushi Sun & Cu Nguyen, “A Highly Sensitive System For Aircraft Crack Detection – The Rotational Remote
Field Eddy Current Probe And Super Sensitive Eddy Current System”, Presented at The 46th Annual NDT Forum,
Montreal, QC, Canada, September 22 -25, 2003.

[10]. Yushi Sun, Tianhe Ouyang, Jie Long, Denis Roach “Rotational Remote-Field Eddy Current Method For
Detecting Cracks Under Raised Head Fasteners”, Presented at The 7th FAA/DoD/NASA Joint Conference on Aging
Aircraft, New Orleans, LA, September 7-11, 2003.

[11]. Yushi Sun, Tianhe Ouyang, Jie Long, Jeff Thompson, Jeff Kollgaard, “Detecting Cracks Under Bushings With
Rotational Remote-Field Eddy Current Probes”, Presented at The 7th FAA/DoD/NASA Joint Conference on Aging
Aircraft, New Orleans, LA, September 7-11, 2003.

[12]. Yushi Sun, Harry Zhu, Tom Moran, John C. Brausch, Kenneth J. LaCivita, Anderson Danny, Thomsen Mark,
Lisa Brasche, and Michael Harper, “Detecting Cracks under Bushings in Aircraft Structures Using FG RFEC &
SSEC Technique”, Presented at ASIP 2004 Conference held in Memphis, TN, November 29 – December 2, 2004.

[13]. Yushi Sun, Dennis Roach, Harry Zhu, “ New Advances in Detecting Cracks in Raised-Head Fastener Holes
Using Rotational Remote Field Eddy Current Technique”, Presented at 2005 ASNT Fall Conference held in
Columbus, OH, October 17-21, 2005.




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Yushi Sun, John C Bausch, Kenneth J. LaCivita and Lt William Sanders, 9th Joint FAA/DoD/NASA Aging Aircraft
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